Leisha Hailey took to Twitter to call for a boycott of the carrier after a flight attendant told them other passengers had complained after witnessing the affection.
Her first tweet said: "I have been discriminated against." She later added, "Since when is showing affection to someone you love illegal?"
Southwest Airlines Co. responded on its website that Hailey was approached "based solely on behavior and not gender." The airline's four-sentence response said passengers were characterizing the behavior as excessive.
A discussion followed on the flight, and the airline said it "escalated to a level that was better resolved on the ground."
Hailey was a musician before joining the cast of the Showtime drama featuring the lives of lesbian friends and lovers living in Los Angeles. She played the character Alice Pieszecki.
The actress and her unidentified girlfriend were on a flight from Baltimore to St. Louis. The kissing occurred in the air and a discussion followed when the plane landed.
Hailey said the encounter between the couple and a flight attendant was recorded.
A message seeking comment from Halley's spokeswoman, Libby Coffey, was not immediately returned.
Southwest's website says it is the official airline of the Gay & Lesbian Alliance Against Defamation.
Herndon Graddick, senior director of programs at GLAAD, said in an email that "GLAAD contacted Southwest to call for additional actions beyond tonight's statement that ensure all customers feel comfortable and welcomed while traveling."
Earlier this month, the Dallas-based airline kicked off Green Day's lead man Billie Joe Armstrong for wearing his pants too low. The Grammy winner was escorted off a plane after failing to follow a flight attendant's directive to pull the pants up.
Southwest also removed director Kevin Smith from a flight last year because he didn't fit properly in a single seat. His first tweet read, "Dear (at)SouthwestAir I know I'm fat, but was (the) captain (...) really justified in throwing me off a flight for which I was already seated?"
Halley is preparing to launch a 21-city tour to promote breast cancer awareness.